53 lines
2.1 KiB
Markdown
53 lines
2.1 KiB
Markdown
---
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categories:
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- Electronics
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tags: [physics, electricity]
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---
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# Current
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> Electrical current is the movement of electrons from negatively charged atoms to negatively charged atoms when an appropriate external force is applied.
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So current is the flow of electrons. Charge is the quantity that flows.
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> The amount of current is the sum of the charges of the moving electrons past a given point.
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## Why current exists
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Current exists because of the [first law of electrostatics](/Electronics/Physics_of_electricity/Coulombs_Laws.md).
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When there is an excess of electrons at one terminal (i.e. negatively charged atoms) and a deficiency of electrons at the other terminal (i.e. positively charged atoms), a \*\*difference of potential\_ exists between the two terminals.
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When the terminals are connected to each other via a conductor (e.g. copper wire) electrons will flow along the conductor. This is provided that there is a source to supply electrons at one end and remove them at the other. We call this force the **voltage source**.
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_The diagram below illustrates the flow of current where the circles are electrons knocking into each other and passing current:_
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> Electrons travel very slowly through a conductor. This is in contrast to their intrinsic motion which of course equal to the speed of light (186, 000 miles per second).
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## Formal expression
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We measure **charge** in Coulombs ($C$). A Coulomb is an aggregate of the charge of thousands of electrons because their individual charge is so small.
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One Coulomb is equal to the charge of $6.24 \cdot 10 ^{18}$ electrons.
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We measure **current** in amps. When one coulomb of charge moves past a point in one second it is called an **ampere** (amp) represented as $A$.
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This relationship is captured in the following equation:
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$$
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I = \frac{Q}{t}
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$$
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- $I$ = current measured in amps
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- $Q$ = quantity of electrical charge measured in coulombs
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- $t$ = time
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### Application
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_Calculate the current in amps if 9 coulombs of charge flow past a point in an electric circuit in 3 seconds._
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$$
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I = \frac{9}{3} \\
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I = 3 A
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$$
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